Blue laser diodes (LDs) have the potential for increasing the storage capacity of optical disks over the densities currently available in compact disk systems based on red laser diodes. Increased storage capacity will open new markets for compact disks in motion picture distribution.
One class of blue emitting elements is based on group III-V nitride films such as GaN epilayers grown on sapphire substrates. To fabricate a laser, a ridge structure is constructed to provide an appropriate optical cavity having parallel mirrors at each end of the cavity. The laser cavity is typically formed by sandwiching an active gain layer between two layers of GaN doped to form n-type and p-type semiconductors. The GaN layers are constructed so as to form a waveguide by depositing the various layers and then etching the stack to form a ridge structure whose vertical walls provide the waveguide. The ends of the waveguide are mirrors that reflect the light generated in the active region back and forth. In GaN based LDs the mirrors are typically formed by cleaving or etching the ends of the waveguide to provide the reflecting surface of the mirror.
The ridge structure discussed above has two problems. First, the structure has poor heat dissipation. The heat generated in the active region must either be dissipated through the substrate or the walls of the ridge structure. The path to the substrate is restricted by the width of the ridge structure; hence, removing heat by transferring the heat to the substrate, which is typically in thermal contact with a heat sink, is difficult.
The second problem with ridge structured devices is the high voltages needed to operate the devices. The p-contact is typically an ohomic contact on the top of the ridge. The resistance of this contact must be overcome to drive the device. To reduce this resistance, the contact needs to have as large an area as possible. However, the available area is limited by the area on the top of the ridge.
Broadly, it is the object of the present invention to provide an improved edge emitting laser diode.
It is a further object of the present invention to provide an edge emitting diode that does not utilize a ridge structure, and hence, avoids the above-described problems.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.